<!DOCTYPE html>
<html lang='en'>

<head>
    <meta charset='UTF-8' />
    <meta name='viewport' content='width=device-width, initial-scale=1.0' />
    <title>Document</title>
    <script src='../js/vue.js'></script>
</head>
<!-- 
            监视属性watch
                1.当被监视的属性变化时，回调函数自动调用，进行相关操作
                2.监视的属性必须存在，才能进行监视！！
                3.监视的两种写法
                    (1).new Vue时传入watch配置
                    (2).通过vm.$watch进行监视
         -->

<body>
    <div id='root'>
        <h2>今天天气很{{info}}</h2>
        <br>
        <button @click="changeWeather">切换天气</button>
    </div>
    <script>
        const vm = new Vue({
            el: '#root',
            data: {
                isHot: true,
                x: 1,
            },
            computed: {
                info() {
                    return this.isHot ? "炎热" : "凉爽"
                }
            },
            methods: {
                changeWeather() {
                    this.isHot = !this.isHot;
                    this.x++;
                },
            },
            watch: {
                /* isHot: {
                     immediate: true, //初始化时让handler调用一下，默认false
                     handler(newValue, oldValue) {
                         console.log("isHot被修改了", newValue, oldValue)
                     }
                 },*/
                info: {
                    immediate: true, // 初始化时让handler调用一下,默认false

                    // handler什么时候调用？当isHot发生改变时
                    handler(newValue, oldValue) {
                        console.log('info被修改了', newValue, oldValue);
                    },
                },
            }
        });
        // 同样能实现监视
        vm.$watch('isHot', {
            immediate: true, // 初始化时让handler调用一下,默认false
            // handler什么时候调用？当isHot发生改变时
            handler(newValue, oldValue) {
                console.log('isHot被修改了', newValue, oldValue);
            },
        }); 
    </script>
</body>

</html>